Britain must not expect to end up with more rights than other countries outside the EU following Brexit negotiations, Italy's Prime Minister Matteo Renzi warned in an interview Thursday.

Renzi also told the BBC that he blamed Britain's former premier David Cameron for June's vote to leave the European Union.

His comments are the latest from a European leader to highlight the tough stance Britain is likely to face in Brexit negotiations when it triggers Article 50, the two-year process for leaving.

"It will be impossible to give to British people more rights than other people out of the EU," Renzi said, speaking in English.

"When David Cameron decided to use the referendum to solve some internal problem in the Conservative party, this was the problem," he added.

Cameron was seen as having called the referendum to try and placate eurosceptics in his party and stem the rise of the anti-EU UK Independence Party.

Renzi added that the vote to leave the EU was "a bad decision" but the result should be respected.

Meanwhile, the Institute for Government think-tank warned that planning for Brexit could cost Britain's government £65 million (75 million euros, $85 million) a year in a new report out Thursday.

It also urged Prime Minister Theresa May to make clear when she plans to activate Article 50.

"In the absence of a clear plan, 'Kremlinology' and off-the-cuff remarks are filling the void," it said.